Recently, in the field of information processing and transmission, attention has been given to an information processing system in which an optical communication system is used for increasing the amount of information transmitted and extending the functions of the system. Accordingly, there is a great demand for optical control devices such as optical switches and optical modulators to control optical signals. Further, there is a demand for optical control devices in which processing is carried out at high speed, the efficiency is high, the dimensions are reduced, and parts are integrated.
As examples of optical control devices used for optical communication which satisfy the demands described above, optical modulators and optical switches are known in which optical waveguide patterns are formed on a crystal substrate, and electrodes and others are arranged on these patterns. Especially, in order to enhance the efficiency and speed, attention is given to a substrate in which a crystal of lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) is used because the crystal is a ferroelectric substance provided with electro-optical effect.
In the case of the optical waveguide device in which a crystal of LiNbO.sub.3 or LiTaO.sub.3 is used as the substrate, a pattern of the optical waveguide is formed in the following manner:
An optical waveguide pattern is formed on a substrate, using a metal such as titanium (Ti), by means of photolithography. When the substrate and pattern are heated to a temperature of 1000.degree. C. to 1050.degree. C., Ti metal on the substrate is thermally diffused, so that an optical waveguide pattern is formed, the width of which is usually several .mu.m to several tens of .mu.m, and the length of which is several mm to several tens of mm.
Since the above substrate, made of LiNbO.sub.3 or LiTaO.sub.3, exhibits a strong pyroelectric effect, an electric charge is generated when the temperature is changed, so that the substrate surface is electrically charged and a high potential is generated on the surface. Accordingly, a large amount of pyroelectric charge tends to accumulate on the metallic pattern (optical waveguide pattern) when the metallic pattern of Ti is heated so as to be diffused onto the substrate surface. As a result, an electric discharge is caused in a portion on the substrate, and the optical wavelength pattern is damaged.
When the rate of increase of the temperature is reduced to provide a gentle temperature change in the case where the substrate is heated, serious problems are not caused. However, in the case where the substrate is heated to a temperature from 1000.degree. C. to 1050.degree. C., taking into consideration that the productivity is enhanced, the substrate is heated, for example, at a rate of 10.degree. C./min. In this case, there is a possibility that an electric discharge is caused between the waveguide patterns by the pyroelectric effect.
In this connection, the following method can be considered. The temperature of the substrate is not quickly increased, and any generated electric charge is neutralized by ions included in the atmosphere. In this way, an excessive amount of electric charge is not accumulated on the waveguide pattern. However, this method is disadvantageous in that a complicated ion supply device is required. Therefore, this method can not be put into practical use.